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Application of CoAPO-5 molecular sieves as heterogeneous catalysts in liquid phase oxidation of alkenes with dioxygen
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier ScienceB.V.
1029
A p p l i c a t i o n of C o A P O - 5 m o l e c u l a r s i e v e s a s h e t e r o g e n e o u s c a t a l y s t s in l i q u i d p h a s e o x i d a t i o n of a l k e n e s w i t h d i o x y g e n H.F.W.J. van Breukelen, J.H.C. van Hooff
M.E.
Gerritsen,
V.M.
Ummels,
J.S.
Broens,
Schuit Institute of Catalysis, Department of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands.
Incorporation of divalent cobalt into the lattice of A1PO-5 yields a catalyst that can be used in the liquid phase epoxidation of (cyclo)alkenes at mild temperatures with dioxygen as oxidant and in the presence of a sacrificial aldehyde.
1. I N T R O D U C T I O N Since the introduction of CoAPO-5 in 1985 [1, 2], a few examples of liquid phase oxidation reactions using this molecular sieve as catalyst have been described in literature. Almost without exception these reactions have a (large) homogeneous contribution of dissolved cobalt to the activity due to leaching of some of the cobalt out of the CoAPO-5 lattice. The reported oxidation of p-cresol into p-hydroxybenzaldehyde was carried out in a strongly alkaline methanol solution [3, 4], while the oxidation of cyclohexane [5-10] and n-hexane [5, 6] were carried out in acetic acid, both causing some destruction of the CoAPO molecular sieve. However no cobalt loss was observed when a substrate to acetic acid ratio of four [5, 6] or when pure cyclohexane was used [10]. In this paper the liquid phase oxidation of (cyclo)alkenes into the corresponding epoxides under Mukaiyama's conditions (atmosphere of dioxygen and a sacrificial aldehyde) and using CoAPO-5 as a heterogeneous catalyst will be reported.
2. E X P E R I M E N T A L
$.1. Synthesis CoAPO-5 was prepared essentially following the procedures described in the patent literature [ 11, 12]. A gel with the following composition: 1.0-x/2 A1203 : 1.02 P205 :x CoO : 1.5 Et3N : 50 H20; 0
1030 was prepared mixing 33.0 g of ortho-phosphoric acid (89 wt.-%, Merck), 130 g of water and the correct amount of cobalt nitrate hexahydrate (Janssen, 97 %), while the correct amount of pseudoboehmite (Condea, 75 wt% A1203) and 22.2 g of triethylamine (Janssen, 99 %) were added under vigorous stirring. The reaction mixture was heated at 443 K for 48 hours in Teflon lined stainless steel autoclaves. After rapid quenching in cool water, the CoAPO-5 was thoroughly washed with water and dried overnight at 353 K. The template was removed from the pores by heating the sample in a flow of dry air at a rate of 5 K min "1 to 823 K and keeping it at the final temperature for five hours. 2.2. C h a r a c t e r i s a t i o n X-ray powder diffraction data of the obtained CoAPO molecular sieves were collected on a Philips PW 7200 X-ray powder diffractometer using Cu Ka radiation. In order to determine the cobalt content of the molecular sieve elemental analysis was performed using a Perkin Elmer 3030 Atomic Absorption Spectrophotometer. Pore volumes were measured by adsorption of n-butane on a Cahn 2000 electrobalance, while Scanning Electron Microscopy (JEOL 840A) was used to determine the agglomerate size. 2.3. Catalytic o x i d a t i o n r e a c t i o n s Typically, an all glass batch reactor was heated at 313 K under atmospheric pressure of dioxygen. It was charged with 50 ml of acetonitrile, 500 ~1 of alkene (about 5 mmol), 500 ~1 of 2-octanone (internal GC standard, 3.2 mmol) and 1000 ~1 of isobutanal (11.0 mmol). All chemicals were purchased from Aldrich or Janssen in their highest purity and used as received. Under magnetic stirring dried (at 393 K) CoAPO-5 (0.1 mmol Co, i.e. 500 mg of CoAPO-5 (1.5 wt.-%)) was added as catalyst. The amounts of substrates and reactants were followed in time by GC analyses (Carlo Erba GC 6000 Vega Series 2 with DB-1 capillary column, injection temperature: 523 K, column temperature 423 K, detector temperature: 523 K).
3. R E S U L T S AND DISCUSSION All CoAPO-5 samples had X-ray diffraction patterns identical to those published in literature. No crystalline impurities could be detected. The weight percentage of cobalt in the samples was between 0 and 2.12 wt.-%, while the pore volumes (n-butane) were higher than 0.090 ml/g. The agglomerate size was approximately 40 ~m. These CoAPO-5 molecular sieves were used as catalysts in the epoxidation of (cyclo)alkenes. A typical progress of the oxidation of cyclohexene using CoAPO-5 (1.21 wt.-%) under standard conditions is displayed in figure 1. The cyclohexene concentration can only be determined correctly at the start, because the GC signal of the formed isobutyric acid could not be separated from the GC signal of cyclohexene.
1031
6
mmol of compound
i
..-I
otyric acid,
.................
. ~ 1 7 6. . . . . . . . . . . . . . . . . .
.............. " .......... ......... ...... I
:'::'-"- ..... '
0
.......
'
T
30
'
CHOL
. ....
-- I-----'-'-"
7.~ ~ - " '
!
60
-''
9 . .........
9 ...........
4b--'
I
--4'
90
'
CHON " ~
~ I
120
'
'
9 ...........
~'~ I
150
'
'
180
t (min)
Fig.1. Conversion of cyclohexene into cyclohexene epoxide, 2-cyclohexen-l-one (CHON) and 2-cyclohexen-l-ol (CHOL) as function of time with CoAPO-5 (1.21 wt.-%) as catalyst (T = 313 K).
3.1. V a r i a t i o n o f t h e s u b s t r a t e Besides cyclohexene a large number of other linear and cyclic alkenes have been tested. In table 1 the used substrates, products, conversions and selectivities are reported. As can be seen all used alkenes react with dioxygen in the presence of a sacrificial aldehyde with the exception of 2-cyclohexen-l-one and cinnamyl alcohol (no reaction could be detected) and 2-cyclohexen-l-ol (besides the expected product also 2-cyclohexen-1-one is formed). Selectivities for the epoxide formation are high, especially for cyclooctene and cyclododecene. 2-octene, 2-hexene and 3-hexene show a higher conversion than 1-octene and 1-hexene. The unsaturated bond of the first three substrates have a higher nucleophilic character than the last two, thereby favouring the oxidation by the electrophilic peracid. Cyclododecene, a molecule too large to enter the pores of CoAPO-5, shows activity for the epoxidation by dioxygen in the presence of isobutyraldehyde. This can be explained by a mechanism by which the sacrificial aldehyde is oxidised inside the pores of the molecular sieve and the alkene is subsequently epoxidised in solution by the so formed peroxo acid as depicted in the scheme on the next page.
1032 Table 1. Used substrates and formed products in epoxidation reaction, conversion (amount of alkene converted) and selectivity towards the epoxide are determined after 3 hours (T = 313 K). Substrate
Product ~ o
o
o
Selectivity (%) 85
0
0
00
?
12
98
~
53
88
~/<~...
30
92
~/~/~/~-~
34
71
~
94
89
O
0
~/~
~~.....oH
Conversion (%) 50
~.~
~ o N
CoAPO ~ RCO3H Solution RCO3H + alkene ~ epoxide + RCO2H RCHO + 02
1033 When the amount of cyclohexene is increased, the rate of formation of the epoxide decreases. This could be caused by the fact that more alkene is adsorbed inside the pores, thus inhibiting the oxidation of the aldehyde to the peroxo acid. 3.2. V a r i a t i o n in the a l d e h y d e The epoxidation reaction has been tested in the absence of a sacrificial aldehyde but then no reaction could be observed. The aldehyde is therefore necessary in this reaction. The cobalt in CoAPO-5 seems not to be possible to activate a]kenes for the oxidation by dioxygen. Besides isobutyraldehyde, propanal and 3,4,5-trimethoxybenzaldehyde have been used as sacrificial aldehydes. The reactivity of isobutyraldehyde was the highest while 3,4,5-trimethoxybenzaldehyde showed no reactivity at all. Also in the case of a cobalt(II) acetate catalysed reaction, 3,4,5-trimethoxybenzaldehyde showed no reactivity at all so at this moment nothing can be said about the place where the oxidation of the aldehyde occurs. Other aldehydes will be tested, but isobutyraldehyde and propanal can serve quite well as sacrificial aldehydes because of their low price. When the amount of aldehyde is increased, the rate of formation of the epoxide is also increased. More aldehyde is adsorbed inside the pores, thus favouring the oxidation of the aldehyde to the peroxo acid. However a maximum rate of formation is reached, probably due to diffusion limitations of the aldehyde into the pores and peracid out of the pores of the molecular sieve. 3.3. V a r i a t i o n of the s o l v e n t Varying the solvent from tetrahydrofuran (THF, no reaction has been observed) via acetonitrile towards tetrachloromethane results in an increase of the rate of formation of the epoxide and also in absolute amount of epoxide formed. THF is a strongly co-ordinating solvent to metal sites, while acetonitrile has less co-ordinating properties and tetrachloromethane has none. THF is also a polar solvent, while tetrachloromethane is apolar. THF is therefore preferentially adsorbed into the pores of the polar structure of the aluminophosphate in comparison to the substrates, while in case of tetrachloromethane the substrates are more polar than the solvent. So the difference in rate of formation can be explained by the difference in coordinating properties of the solvent and polarity. 3.4. Filtration of the catalyst An average conversion of 50 % for the epoxidation of cyclohexene is obtained after 3 hours at 313 K and atmospheric pressure of dioxygen, at which time the isobutyraldehyde is almost completely consumed, therefore no further reaction is possible. When a homogeneous cobalt catalyst (e.g. cobalt(II) acetate, cobalt(II) nitrate, cobalt(II) sulphate or cobalt(II) acetylacetonate) is used with the same amount of cobalt present as in the CoAPO-5, the maximum conversion is reached in less than 3 hours. Except for cobalt(II) acetylacetonate only 30 minutes is
1034 necessary to reach maximum conversion using a homogeneous catalyst, while no further reaction could be observed after this time. Table 2. CoAPO-5 catalysed oxidation of cyclohexene in the liquid phase with 02. Conversions and selectivities are determined after three hours (T = 313 K). Selectivity Catalyst Conversion 92 % CoAPO-5 (1.21 wt% Co) 44 % CoAPO-5 (1.21 wt% Co)/none s 7 % 93 % 71% CoAc2.4H20 50 % None 12 % 86 % Catalyst removed alter 10 minutes by filtration. Results of a series of measurements in the epoxidation of cyclohexene are shown in table 2. At the reported conversion levels for CoAPO-5 and homogeneous catalysed reactions, the amount of isobutanal has been consumed completely, so there is conversion of isobutanal into a species, which can not epoxidise cyclohexene. The selectivity of the CoAPO-5 catalysed reaction (92 %), which is comparable to the selectivity for the oxidation of cyclohexene catalysed by cobalt-resin [13], is higher than for the homogeneous catalysed reaction (71%). In the first ten minutes of the CoAPO-5 catalysed reaction no loss of cobalt is found, in contrast with the oxidation of cinnamyl alcohol catalysed by VAPO-5, where a metal loss from the catalyst is present from the beginning of the reaction [14]. Also filtration between 30 and 60 minutes does not result in a cobalt loss. With AAS only at the lowest detectable level (less than 0.1 ppm in solution) cobalt was observed after filtration of the catalyst after 75 minutes. It is not certain if it is really dissolved cobalt because particles smaller than 0.45 ~m can not be separated from solution by the filters used for the separation of the catalyst. Also matrix effects (presence of organic compounds in sample) could change the AAS signal. The continuing activity after removal of the catalyst can be explained by the fact that there could still be radicals in solution, which can continue to form peroxo radicals. According to the mechanism proposed by Chou et al. [13] isobutanal is turned via reactive intermediates into isobutyric acid. This acid could be the cause for the dissolution of cobalt out of the lattice.
4. CONCLUSIONS It has been shown that CoAPO-5 is a usable solid catalyst for the selective, liquid phase oxidation of (cyclo)alkenes to the corresponding epoxide, using dioxygen as the terminal oxidant and a sacrificial aldehyde. However more research at a possible leaching of cobalt out of the lattice is necessary.
1035 REFERENCES [1] [2l [3l
C./~ Messina, B.M. Lok and E.M. Flanigen, U.S. Patent, 4 544 143, 1985. S.T. Wilson and E.M. Flanigen, Eur. Pat. Appl., 132 780, 1985. M.P.J. Peeters, M. Busio, P. Leijten and J.H.C. van Hooff, Appl. Catal. A: General, 1994, 118, 51-62. [41 J. Dakka, R.A. Sheldon, Neth. Patent NL 9200968 (1992). [51 B. Kraushaar-Czarnetzki, W.G.M. Hoogervorst and W.H.J. Stork, Stud. Surf. Sci. Catal., 1994, 84, 1869-1876. [61 B. Kraushaar-Czarnetzki, W.G.M. Hoogervorst, Eur. Pat. Appl. 0 519 569
(1992). [71 S.-S. Lin and H.-S. Weng, Appl. Catal. ,4: General, 1993, 105, 289-309. [81 S.-S. Lin and H.-S. Weng, Appl. Catal. A: General, 1994, 118, 21-31. [91 S.-S. Lin and H.-S. Weng, J. Chem. Eng. Jpn., 1994, 27, 211-215. [10] D.L. Vanoppen, D.E. de Vos, M.J. Genet, P.G. Rouxhet, P.A. Jacobs, Angew. Chem. Int. Ed. Engl. 34 (1995), 560-563. [11] S.T. Wilson and E.M. Flanigen, U.S. Patent, 4 567 029, 1986. [12] E.M. Flanigen, B.M.T. Lok, R.L. Patton and S.T. Wilson, U.S. Patent, 4 759 919, 1988. [13] T.C. Chou, L.C. Chen, J. Chin. Inst. Chem. Eng. 22 (1991), 209-218. [14] M.J. Haanepen, Thesis Eindhoven University of Technology, 1996.
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier ScienceB.V.
1029
A p p l i c a t i o n of C o A P O - 5 m o l e c u l a r s i e v e s a s h e t e r o g e n e o u s c a t a l y s t s in l i q u i d p h a s e o x i d a t i o n of a l k e n e s w i t h d i o x y g e n H.F.W.J. van Breukelen, J.H.C. van Hooff
M.E.
Gerritsen,
V.M.
Ummels,
J.S.
Broens,
Schuit Institute of Catalysis, Department of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands.
Incorporation of divalent cobalt into the lattice of A1PO-5 yields a catalyst that can be used in the liquid phase epoxidation of (cyclo)alkenes at mild temperatures with dioxygen as oxidant and in the presence of a sacrificial aldehyde.
1. I N T R O D U C T I O N Since the introduction of CoAPO-5 in 1985 [1, 2], a few examples of liquid phase oxidation reactions using this molecular sieve as catalyst have been described in literature. Almost without exception these reactions have a (large) homogeneous contribution of dissolved cobalt to the activity due to leaching of some of the cobalt out of the CoAPO-5 lattice. The reported oxidation of p-cresol into p-hydroxybenzaldehyde was carried out in a strongly alkaline methanol solution [3, 4], while the oxidation of cyclohexane [5-10] and n-hexane [5, 6] were carried out in acetic acid, both causing some destruction of the CoAPO molecular sieve. However no cobalt loss was observed when a substrate to acetic acid ratio of four [5, 6] or when pure cyclohexane was used [10]. In this paper the liquid phase oxidation of (cyclo)alkenes into the corresponding epoxides under Mukaiyama's conditions (atmosphere of dioxygen and a sacrificial aldehyde) and using CoAPO-5 as a heterogeneous catalyst will be reported.
2. E X P E R I M E N T A L
$.1. Synthesis CoAPO-5 was prepared essentially following the procedures described in the patent literature [ 11, 12]. A gel with the following composition: 1.0-x/2 A1203 : 1.02 P205 :x CoO : 1.5 Et3N : 50 H20; 0
1030 was prepared mixing 33.0 g of ortho-phosphoric acid (89 wt.-%, Merck), 130 g of water and the correct amount of cobalt nitrate hexahydrate (Janssen, 97 %), while the correct amount of pseudoboehmite (Condea, 75 wt% A1203) and 22.2 g of triethylamine (Janssen, 99 %) were added under vigorous stirring. The reaction mixture was heated at 443 K for 48 hours in Teflon lined stainless steel autoclaves. After rapid quenching in cool water, the CoAPO-5 was thoroughly washed with water and dried overnight at 353 K. The template was removed from the pores by heating the sample in a flow of dry air at a rate of 5 K min "1 to 823 K and keeping it at the final temperature for five hours. 2.2. C h a r a c t e r i s a t i o n X-ray powder diffraction data of the obtained CoAPO molecular sieves were collected on a Philips PW 7200 X-ray powder diffractometer using Cu Ka radiation. In order to determine the cobalt content of the molecular sieve elemental analysis was performed using a Perkin Elmer 3030 Atomic Absorption Spectrophotometer. Pore volumes were measured by adsorption of n-butane on a Cahn 2000 electrobalance, while Scanning Electron Microscopy (JEOL 840A) was used to determine the agglomerate size. 2.3. Catalytic o x i d a t i o n r e a c t i o n s Typically, an all glass batch reactor was heated at 313 K under atmospheric pressure of dioxygen. It was charged with 50 ml of acetonitrile, 500 ~1 of alkene (about 5 mmol), 500 ~1 of 2-octanone (internal GC standard, 3.2 mmol) and 1000 ~1 of isobutanal (11.0 mmol). All chemicals were purchased from Aldrich or Janssen in their highest purity and used as received. Under magnetic stirring dried (at 393 K) CoAPO-5 (0.1 mmol Co, i.e. 500 mg of CoAPO-5 (1.5 wt.-%)) was added as catalyst. The amounts of substrates and reactants were followed in time by GC analyses (Carlo Erba GC 6000 Vega Series 2 with DB-1 capillary column, injection temperature: 523 K, column temperature 423 K, detector temperature: 523 K).
3. R E S U L T S AND DISCUSSION All CoAPO-5 samples had X-ray diffraction patterns identical to those published in literature. No crystalline impurities could be detected. The weight percentage of cobalt in the samples was between 0 and 2.12 wt.-%, while the pore volumes (n-butane) were higher than 0.090 ml/g. The agglomerate size was approximately 40 ~m. These CoAPO-5 molecular sieves were used as catalysts in the epoxidation of (cyclo)alkenes. A typical progress of the oxidation of cyclohexene using CoAPO-5 (1.21 wt.-%) under standard conditions is displayed in figure 1. The cyclohexene concentration can only be determined correctly at the start, because the GC signal of the formed isobutyric acid could not be separated from the GC signal of cyclohexene.
1031
6
mmol of compound
i
..-I
otyric acid,
.................
. ~ 1 7 6. . . . . . . . . . . . . . . . . .
.............. " .......... ......... ...... I
:'::'-"- ..... '
0
.......
'
T
30
'
CHOL
. ....
-- I-----'-'-"
7.~ ~ - " '
!
60
-''
9 . .........
9 ...........
4b--'
I
--4'
90
'
CHON " ~
~ I
120
'
'
9 ...........
~'~ I
150
'
'
180
t (min)
Fig.1. Conversion of cyclohexene into cyclohexene epoxide, 2-cyclohexen-l-one (CHON) and 2-cyclohexen-l-ol (CHOL) as function of time with CoAPO-5 (1.21 wt.-%) as catalyst (T = 313 K).
3.1. V a r i a t i o n o f t h e s u b s t r a t e Besides cyclohexene a large number of other linear and cyclic alkenes have been tested. In table 1 the used substrates, products, conversions and selectivities are reported. As can be seen all used alkenes react with dioxygen in the presence of a sacrificial aldehyde with the exception of 2-cyclohexen-l-one and cinnamyl alcohol (no reaction could be detected) and 2-cyclohexen-l-ol (besides the expected product also 2-cyclohexen-1-one is formed). Selectivities for the epoxide formation are high, especially for cyclooctene and cyclododecene. 2-octene, 2-hexene and 3-hexene show a higher conversion than 1-octene and 1-hexene. The unsaturated bond of the first three substrates have a higher nucleophilic character than the last two, thereby favouring the oxidation by the electrophilic peracid. Cyclododecene, a molecule too large to enter the pores of CoAPO-5, shows activity for the epoxidation by dioxygen in the presence of isobutyraldehyde. This can be explained by a mechanism by which the sacrificial aldehyde is oxidised inside the pores of the molecular sieve and the alkene is subsequently epoxidised in solution by the so formed peroxo acid as depicted in the scheme on the next page.
1032 Table 1. Used substrates and formed products in epoxidation reaction, conversion (amount of alkene converted) and selectivity towards the epoxide are determined after 3 hours (T = 313 K). Substrate
Product ~ o
o
o
Selectivity (%) 85
0
0
00
?
12
98
~
53
88
~/<~...
30
92
~/~/~/~-~
34
71
~
94
89
O
0
~/~
~~.....oH
Conversion (%) 50
~.~
~ o N
CoAPO ~ RCO3H Solution RCO3H + alkene ~ epoxide + RCO2H RCHO + 02
1033 When the amount of cyclohexene is increased, the rate of formation of the epoxide decreases. This could be caused by the fact that more alkene is adsorbed inside the pores, thus inhibiting the oxidation of the aldehyde to the peroxo acid. 3.2. V a r i a t i o n in the a l d e h y d e The epoxidation reaction has been tested in the absence of a sacrificial aldehyde but then no reaction could be observed. The aldehyde is therefore necessary in this reaction. The cobalt in CoAPO-5 seems not to be possible to activate a]kenes for the oxidation by dioxygen. Besides isobutyraldehyde, propanal and 3,4,5-trimethoxybenzaldehyde have been used as sacrificial aldehydes. The reactivity of isobutyraldehyde was the highest while 3,4,5-trimethoxybenzaldehyde showed no reactivity at all. Also in the case of a cobalt(II) acetate catalysed reaction, 3,4,5-trimethoxybenzaldehyde showed no reactivity at all so at this moment nothing can be said about the place where the oxidation of the aldehyde occurs. Other aldehydes will be tested, but isobutyraldehyde and propanal can serve quite well as sacrificial aldehydes because of their low price. When the amount of aldehyde is increased, the rate of formation of the epoxide is also increased. More aldehyde is adsorbed inside the pores, thus favouring the oxidation of the aldehyde to the peroxo acid. However a maximum rate of formation is reached, probably due to diffusion limitations of the aldehyde into the pores and peracid out of the pores of the molecular sieve. 3.3. V a r i a t i o n of the s o l v e n t Varying the solvent from tetrahydrofuran (THF, no reaction has been observed) via acetonitrile towards tetrachloromethane results in an increase of the rate of formation of the epoxide and also in absolute amount of epoxide formed. THF is a strongly co-ordinating solvent to metal sites, while acetonitrile has less co-ordinating properties and tetrachloromethane has none. THF is also a polar solvent, while tetrachloromethane is apolar. THF is therefore preferentially adsorbed into the pores of the polar structure of the aluminophosphate in comparison to the substrates, while in case of tetrachloromethane the substrates are more polar than the solvent. So the difference in rate of formation can be explained by the difference in coordinating properties of the solvent and polarity. 3.4. Filtration of the catalyst An average conversion of 50 % for the epoxidation of cyclohexene is obtained after 3 hours at 313 K and atmospheric pressure of dioxygen, at which time the isobutyraldehyde is almost completely consumed, therefore no further reaction is possible. When a homogeneous cobalt catalyst (e.g. cobalt(II) acetate, cobalt(II) nitrate, cobalt(II) sulphate or cobalt(II) acetylacetonate) is used with the same amount of cobalt present as in the CoAPO-5, the maximum conversion is reached in less than 3 hours. Except for cobalt(II) acetylacetonate only 30 minutes is
1034 necessary to reach maximum conversion using a homogeneous catalyst, while no further reaction could be observed after this time. Table 2. CoAPO-5 catalysed oxidation of cyclohexene in the liquid phase with 02. Conversions and selectivities are determined after three hours (T = 313 K). Selectivity Catalyst Conversion 92 % CoAPO-5 (1.21 wt% Co) 44 % CoAPO-5 (1.21 wt% Co)/none s 7 % 93 % 71% CoAc2.4H20 50 % None 12 % 86 % Catalyst removed alter 10 minutes by filtration. Results of a series of measurements in the epoxidation of cyclohexene are shown in table 2. At the reported conversion levels for CoAPO-5 and homogeneous catalysed reactions, the amount of isobutanal has been consumed completely, so there is conversion of isobutanal into a species, which can not epoxidise cyclohexene. The selectivity of the CoAPO-5 catalysed reaction (92 %), which is comparable to the selectivity for the oxidation of cyclohexene catalysed by cobalt-resin [13], is higher than for the homogeneous catalysed reaction (71%). In the first ten minutes of the CoAPO-5 catalysed reaction no loss of cobalt is found, in contrast with the oxidation of cinnamyl alcohol catalysed by VAPO-5, where a metal loss from the catalyst is present from the beginning of the reaction [14]. Also filtration between 30 and 60 minutes does not result in a cobalt loss. With AAS only at the lowest detectable level (less than 0.1 ppm in solution) cobalt was observed after filtration of the catalyst after 75 minutes. It is not certain if it is really dissolved cobalt because particles smaller than 0.45 ~m can not be separated from solution by the filters used for the separation of the catalyst. Also matrix effects (presence of organic compounds in sample) could change the AAS signal. The continuing activity after removal of the catalyst can be explained by the fact that there could still be radicals in solution, which can continue to form peroxo radicals. According to the mechanism proposed by Chou et al. [13] isobutanal is turned via reactive intermediates into isobutyric acid. This acid could be the cause for the dissolution of cobalt out of the lattice.
4. CONCLUSIONS It has been shown that CoAPO-5 is a usable solid catalyst for the selective, liquid phase oxidation of (cyclo)alkenes to the corresponding epoxide, using dioxygen as the terminal oxidant and a sacrificial aldehyde. However more research at a possible leaching of cobalt out of the lattice is necessary.
1035 REFERENCES [1] [2l [3l
C./~ Messina, B.M. Lok and E.M. Flanigen, U.S. Patent, 4 544 143, 1985. S.T. Wilson and E.M. Flanigen, Eur. Pat. Appl., 132 780, 1985. M.P.J. Peeters, M. Busio, P. Leijten and J.H.C. van Hooff, Appl. Catal. A: General, 1994, 118, 51-62. [41 J. Dakka, R.A. Sheldon, Neth. Patent NL 9200968 (1992). [51 B. Kraushaar-Czarnetzki, W.G.M. Hoogervorst and W.H.J. Stork, Stud. Surf. Sci. Catal., 1994, 84, 1869-1876. [61 B. Kraushaar-Czarnetzki, W.G.M. Hoogervorst, Eur. Pat. Appl. 0 519 569
(1992). [71 S.-S. Lin and H.-S. Weng, Appl. Catal. ,4: General, 1993, 105, 289-309. [81 S.-S. Lin and H.-S. Weng, Appl. Catal. A: General, 1994, 118, 21-31. [91 S.-S. Lin and H.-S. Weng, J. Chem. Eng. Jpn., 1994, 27, 211-215. [10] D.L. Vanoppen, D.E. de Vos, M.J. Genet, P.G. Rouxhet, P.A. Jacobs, Angew. Chem. Int. Ed. Engl. 34 (1995), 560-563. [11] S.T. Wilson and E.M. Flanigen, U.S. Patent, 4 567 029, 1986. [12] E.M. Flanigen, B.M.T. Lok, R.L. Patton and S.T. Wilson, U.S. Patent, 4 759 919, 1988. [13] T.C. Chou, L.C. Chen, J. Chin. Inst. Chem. Eng. 22 (1991), 209-218. [14] M.J. Haanepen, Thesis Eindhoven University of Technology, 1996.